As for a headlight of a vehicle, a cheap and stable light source is required. In particular, to implement a stable light source, it is desirable to light stably without illuminance variation. Since illuminance is in proportion to power, a general method of achieving stable lighting is to supply constant power. However, owing to individual differences in characteristics of discharge lamps, or to variations in the life due to electrode wear or in pressure conditions in a light-emitting tube, since the voltage of a discharge lamp varies, a technique is required to achieve constant power control independently of the discharge lamp voltage.
As a prior art for achieving the constant power control of the discharge lamp, there is Patent Document 1. The prior art detects output voltage, calculates a target current=target power/output voltage, and carries out feedback using the target current calculated, thereby achieving constant power control accurately with a configuration simpler than that when carrying out feedback using power. In addition, by providing a delay circuit for the target current output, it alters the target current by a time constant comparable to a transient change in the discharge lamp voltage, thereby preventing power oscillation.